Using a Field Mixer to get Better Audio

Whether you’re shooting with a DSLR with sub par audio controls or you need more inputs than your camcorder has, a field mixer can help expand your audio possibilities and get you better quality audio.

Video Transcript

Getting good audio can be a challenge.  But utilizing the right tools can give you versatile options for capturing great sound that really enhances your video.  Whether you’re shooting with a DSLR with sub par audio controls, or you need to use more inputs than your camcorder has, a field mixer can help expand your audio possibilities, and get you better quality audio.

In this training video, you’ll learn all about field mixers, and we’ll show you how to get audio in, set input levels, route your signal, get audio out, properly calibrate a mixer and recorder, and monitor your signal to ensure great results.
Using a field mixer effectively can ensure that the audio on your next project, really shines.

Before we dive into field mixers, you might be asking yourself, do I actually need to use one.  A field mixer is a great tool to increase the number of inputs you can record.  Even the best DSLR cameras typically have 1 audio input, and camcorders typically have 2. A field mixer is a must if the number of audio sources you need to record exceeds the number of recording inputs you have available.  A quality field mixer will have better pre-amps and more control options that give you better quality sound. If you have the luxury of a dedicated sound operator, a field mixer gives them full control over recording and monitoring the audio without hindering the cinematographer.

The key to maximizing your audio quality, is knowing how to use a field mixer properly, so lets start at the beginning, and talk about getting your audio sources into the mixer.
The most typical way to get audio into a field mixer is using XLR inputs.  You’ll find mixers with as few as one and as many as 6 inputs. You may also find mixers with 1 / 8 " inputs.  These can accommodate mics with 1 / 8 " outputs, such as wired lavaliers or wireless systems.

Each XLR or 1 / 8 " input will have important switches associated with them that you need to set properly, and the configuration may vary from mixer to mixer. Some mixers will have a dedicated switch labeled mic and line. In this case, the mic setting should be used for any microphone you connect.  The line switch would be used for a feed from and audio mixer being fed at line level, or other audio devices that produce a line level signal. The settings we discuss in this tutorial will refer to mic signals coming into the mixer.  With a two way MIC/LINE switch, you’re likely to find a dedicated switch to turn phantom power on or off for each individual input. 
This should be set to off for dynamic mics, and condenser mics that get phantom power from a dedicated battery or wireless transmitter.  The switch should be to on for standard condenser mics or hard wired lavalier mics without their own source of power.

Some mixers will also have a switch to select 48V or 12V for phantom power.  Most mics use 48V, while a limited number of older mics use 12. Other mixers may have a three way switch that has a label like mic, line, and phantom or maybe even dynamic, line, condenser.  On these types of mixers, you should choose dynamic for any mics that don’t require phantom power, and condenser or phantom to send phantom power to any mics that do require power. 

It’s extremely important to match the switch position to the type of input that’s coming into the mixer.  Many mixers adjust the signal level coming into the mixer accordingly to get a proper level into the mixer.

Okay, you’ve got your audio source into the mixer, you’ve got the input selector and phantom power set properly, and now it’s time to dial in your input levels. It’s important to note that some mixers have a dedicated “master” fader to control the outgoing level of your signal. If your mixer has this, you’ll want to determine what the detent or unity position is.  This may be labeled as u or the knob may even click into position.  If it’s not clear, check your manual or contact the manufacturer to find out the ideal position for peak performance.  Once you’ve got this set.  Leave it in that position.

A key component of any field mixer is the ability to monitor your audio levels.  this may be an array of lights, a digital readout, or a single indicator with multiple colors. In general , green is safe, orange or yellow is near clipping, and red is clipping, but be sure to check the manual for your mixer so you know what you’re looking for.  Analog signals can go over 0 db without catastrophic effects, but digital recorders clip at any reading above zero, so be careful.  The goal is to get the healthiest signal you can without clipping your signal. To help you avoid clipping, nearly every field mixer has is a limiter.

A limiter will suppress audio over a specified level..  Some models have a factory preset such as 0db, while others may have an adjustable threshold.  Some mixers have a switch to turn the limiter on or off, while others won’t have to capability to disengage it.

There is debate about best practices regarding the use of limiters.  Some pros swear by letting the audio limiter be engaged often, because it allows for an overall higher recording level, while others claim that the limiter has a noticeable effect on the recording. In the end, the choice probably depends largely on the mixer you're using, and how much control you have to manipulate your audio in post.

While some high end mixers have a lot of options and dials to get a good level, most mid-range mixers have at least two adjustments. Channel Faders, and Gain. Channel faders are typically the big round knobs that can be adjusted from around 7o clock to 5 o'clock.  The goal with most mixers is to keep these knobs somewhere between 12 and 3 o'clock when possible. If you have your fader halfway between, and you’re level is way too low, or way too high, it’s time to adjust the gain switch or dial. The gain switch or dial boosts or reduces the signal before it goes into the channel fader.
A two position gain switch will typically have a high and low setting. If you’re signal is low, change it to high, and if it’s too hot, change it to low. if you’ve got a dial, adjust the dial clockwise to increase the gain, and counterclockwise to decrease the gain. Again, the goal is to keep your channel faders between 12 and 3 if possible.

Once you’ve got a nice level, it’s time to fine tune your audio with some detailed control.  If you’re recording a person speaking on location, the background noise in a shot can be distracting.  Some field mixers have a High Pass or Low Cut tool to helps reduce the volume level of the background, without cutting into the frequencies of the voice.  This may be a dial that can be fine tuned, or a 2 or three way switch with presets.  Common presets fall near 20 hertz, 80 hertz, and 150 herts. 20 hertz is a safe bet, but also not overly effective with a loud background.  80 hertz is a good bet if you’re capturing a male voice, and 150 hertz can work when capturing a female voice.  Be sure to toggle the switch or tune the dial to the different settings, to ensure you’re not cutting out a significant part of the speakers voice.  If you’ve got a dial, start at the lowest setting, and increase it as much as you can without affecting the voice you’re recording.

Once you’ve got healthy levels from your audio sources, it’s time to route your audio to get maximum flexibility in post.  If you’re only using one mic, then routing won’t be much help, but if even if you’ve got a recorder with one stereo input, like a DSLR, routing your signals properly can help keep audio tracks isolated from each other, so they can be adjusted independently in post production.

Let’s take a two mic setup with a simple two channel mixer being fed into a DSLR as an example.  We’ve got a shotgun mic going into channel 1, and a dynamic mic going into channel 2.  On many 2 channel mixers, one XLR input will be designated as LEFT and the other will be designated as RIGHT.  Now we’ll look for a switch that controls how the audio is sent to the recorder.  This switch should be labeled mono and stereo or in some cases m and s.  

Putting the switch to stereo will keep the left channel routed to the left output, and the right channel routed to the right output.  This results in two distict signals that can be individually manipulated in post. On the 2 channel mixer, putting the output switch to mono will result in an input being sent out to both the left and right channels. This is an appropriate setting if you only have one audio source coming in, and you want it to be recorded on both the left and right channel.On a mixer with more than two channels,each channel will have 3 possible pan settings.  Left, Center, and Right.

The center position would send the input out both channels, so in this case, we would route channel one to the left, and channel 2 to the right by changing the switch to the correct position. On some mixers, you’d still need to select stereo output to get the desired result, while other mixers won’t have a switch, and output in stereo at all times.

If you have more than two audio sources, you won’t be able to get an isolated track for each source.  In these cases, route mics with similar characteristics to the same channel, and be sure to get the levels well balanced between the two.
When you’ve got your audio routed properly, it’s time to send off your signal to the recording device or camera.  There are two main ways which this is done, via XLR outputs, or an 1 / 8 " output.  Each channel may have a switch to determine the intensity of the signal you want to send, depending on what type of input you’re recording device takes.  In some cases there may be two separate 1 / 8 " outputs, one sending line level, and the other sending mic level.  Just be sure that whichever you choose, you set the recorder to the same input setting.

To set proper levels on your recorder, look for a switch labeled tone oscilator or osc, and turn it on.  This will generate a standard tone and send it to the mixer outputs.  Adjust the level on your recorder until the level is at 0, or just a hair below it for a little headroom.  Now the meters on your mixer should match the levels being recorded.  You may have to disengage the alarm setting and limiter on your recording device to get proper levels. If your mixer doesn’t have tone,
do mic tests and adjust the recorder input level until it matches the reading on your mixer.

One fantastic feature of field mixers is the ability to monitor either the sound coming into the mixer, or an output from the recording device. This is typically achieved by running a stereo cable from the 1 / 8 " headphone or monitor out jack on the recorder and plugging it into the 1 / 8 " inch input on the field mixer.  This input will be labeled something like tape return, monitor in, or rtn.

Once you’ve got the cable hooked up, look for a switch on the mixer labeled something like mix/tape return, direct/tape, output/return, or play/rec When the switch is on mix, direct, output, or rec you’ll hear the audio from the mixer, when the switch is on tape, return,or play you’ll hear the audio output of the recorder.

If you’ve got the capability to send tone, turn it on, and set the switch to tape or return.  Then adjust the monitor output level of the recording device until the return signal matches the mixer signal volume. If you don’t have tone, do mic tests and switch back and forth between the mixer and the recorder.  Adjust the monitor volume of the recorder until the volume levels are identical. The ability to check the tape return gives you the confidence that the signal sounds as good on the recording as it does on the mixer. To adjust the level of your headphones, you can use the headphone or monitor knob.  Keep in mind, adjusting this has no effect on the levels being sent into and out of your mixer or recorder.

If you’re stuck with a DSLR that doesn’t have manual audio controls, some field mixers can minimize the hiss by dampening the auto gain control on your camera.  Look for a switch labeled “disable AGC”.  Activating this will send a 20Hz tone via one of the channels that will reduce the huge swings of automatic gain control.  The results won’t be anywhere near as good as manual control, but it is an improvement compared to AGC being fully engaged.  Just don’t forget to filter out the 20Hz tone in post.

Understanding how an audio signal flows from an audio source through a field mixer, out to a recorder and even back into your mixer is a fundamental building block to capturing great audio.  With proper mic technique and a little technical know-how, you can capture pristine audio on your next project.  Thanks for watching.